Why does a MixRGB node rotate my ColorRamp/mapping coordinates?
In both your cases, (with and without your Mix RGB node) you are casting a 3-component vector to a 1-component scalar Fac input to the Color Ramp node. But Blender nodes do this differently for RGB Colors, and XYZ Vectors.
The cast from Color to scalar is done with an implicit RGB to BW node; i.e. it uses the luminance of the color as its scalar result. The cast from Vector to scalar is done by taking the simple mean of the XYZ elements.
Since the Position output of your Voronoi node has a range of R and G values across the piece, varying exactly with X and Y in whatever texture-space you are using,  that explains the non-uniform interpretation of the colors as scalars across X and Y.
You could use a home-made 'Mix Vector' group, which would output a Vector, not a Color:

..but it would probably be better to find a way of achieving your desired effect without using the cast at all.